Hydrocarbon fuels, such as diesel, gasoline and natural gas, have generally a sulfur content that in most cases needs to be reduced of environmental reasons and/or because sulfur is a potent poison for catalysts and catalytic processes.
Sulfur is present in hydrocarbon fuels in the form of a variety of sulfur compounds. The sulfur can be removed from the fuel in an industrial hydrodesulfurization process (HDS) before the hydrocarbon fuel comes into actual use, such as in combustion or reformation. HDS may be suitable for large-scale industrial processes where large quantities of hydrogen are available, but it is a costly and complicated process and therefore not suitable in other applications, such as automotive and fuel cell applications.
Another method is to remove sulfur after a fuel reforming step, such as steam reforming or partial oxidation, in which step the fuel is catalytically reformed into smaller hydrocarbons and hydrogen, and in which the sulfur is converted into H2S. This method is relatively effective and useful in many situations. H2S can readily be removed from a gaseous stream by passing the sulfur-containing gas over a material that can react with the sulfur, such as ZnO, and thereby purge the gas stream of sulfur. However, this method has the disadvantage that the catalyst in the reactor will suffer from poisoning which results in short lifetimes for the catalytic system.
U.S. 2003/0188475 describes an example of a fuel reforming system where the sulfur trap has been incorporated after the catalytic reformer. In the disclosed system the fuel is initially vaporized and then catalytically converted in an autothermal reformer into a hydrogen rich gas before passed over the sulfur trap. The product is then fed via a water gas shift reactor and a catalytic preferential oxidation reactor to a fuel cell.
To avoid or at least reduce the problems related to sulfur contamination of catalysts, focus has generally been set on developing catalysts that are more resistant to sulfur or catalysts that are less expensive so that each replacement of poisoned catalyst becomes less costly.